For many years, drum brakes were the most predominant type of brakes used in automotive and other motor vehicle applications. In recent years, however, disc brakes have become more and more widely used. In some cases, disc brakes have been used on only the front wheels of the vehicle and drum brakes used on the rear wheels, but disc brakes are now being used on both front and rear brakes with increasing frequency.
Although disc brakes provide many advantages over drum brakes, one limitation they have relates to parking brake or emergency brake applications. Unlike drum brakes, which can readily double as parking brakes, disc brakes require special modifications in order to perform that function. This limitation of disc brakes is not typically a problem on a vehicle with front disc brakes and rear drum brakes because the rear drum brakes can double as the parking brake. When both front and rear brakes are disc brakes, however, the disc brakes must be modified in order to perform the parking brake function.
One such disc brake configuration involves adapting the disc brake caliper to include components that will compress the piston and brake pads against the brake disk when the parking brake is actuated. This approach increases the complexity of the caliper mechanism and can have problems with reliability. Also, the use of disc brakes for parking can change the static/dynamic friction ratio of the disc friction material, which can lead to undesirable noise generation. An alternative modification of disc brakes to function as parking brakes is known as the drum-in-hat approach. With this approach, a small brake drum is incorporated into the hat section of the disc brake rotor. Drum-in-hat brake assemblies are well-known in the art and are described, for example in U.S. Pat. Nos. 5,180,037, 5,385,216 and 5,529,149, the disclosures of which are incorporated herein by reference. When the parking brake is actuated, a small brake shoe is applied to engage the drum and prevent the wheel from moving. The drum-in-hat approach for parking brakes with disc brake assemblies has become a popular approach due to its reliability and low cost. One problem experienced by drum-in-hat disc brake assemblies has been degradation of the drum brake shoe friction material due to heat buildup in the brake assembly. This problem is particularly severe in heavy duty brakes used on vehicles with gross vehicle weights in excess of 4000 pounds, and is quite surprising as it can occur from repeated application of the disc brakes without the parking brake even being actuated.
Traditional drum brake shoe friction materials are typically rolled into a sheet of friction material that can be cured and cut into brake segments. While this is an efficient way of manufacturing friction material and provides satisfactory friction materials for conventional drum brakes, the liquid resins required for this process have been found to be insufficient to meet the stringent heat-resistance demands of drum-in-hat brake shoes. Moreover, even many conventional solid phenolic resins typically used for compression-molded disc brake pads are unable to withstand sustained temperatures in excess of 450.degree. F., resistance to which has now been found to be necessary in order to meet the demanding standards for these severe drum-in-hat brake shoe applications.